This invention relates to a management system for use in an integrated services digital network (ISDN) to manage a plurality of terminal endpoint identifiers (TEI) which are assigned to a plurality of terminal equipment (TE) units.
An integrated services digital network of the type described comprises a digital exchange connected to a plurality of network termination (NT) units through digital subscriber lines. To each of the network termination units, a subscriber bus or another network termination unit is extended to at least one socket element. Each socket can be used in common to different kinds of terminal equipment (TE) units, such as a telephone, a personal computer, a facsimile device, or the like. With this structure, each user communicate with other devic through the integrated services digital network by selectively connecting each of the terminal equipment units to the socket. Such communication is carried out by the use of a pair of information transfer channels or communication channels (called B-channels) and a control channel (called D-channel) which are multiplexed in a time division fashion. Thus, it is possible to favorably offer a wide variety of services to the users.
In order to enable the above-mentioned communication of various kinds of terminal equipment units, a protocol is prescribed in connection with an interface between users and such an integrated services digital network by recommendations of International Telegraph and Telephone Consultative Communication (CCITT). More specifically, a protocol for such an interface is hierarchically divided into first, second, and third layer protocols which define a procedure between terminal equipment units and network termination (NT) units, a procedure for a D-channel link access, and a procedure for call control from a beginning of communication to an end thereof, respectively.
The following description will be primarily directed to the protocol for the second layer protocol, which is determined for accessing a link on the D-channel and which may be called a link access procedure on the D-channel (LAPD). Inasmuch as various kinds of the terminal equipment units can be selectively connected to a single socket in the integrated services digital network, it is necessary to identify whether or not such a terminal equipment unit is connected to the socket each time when communication is carried out throught the socket.
According to the link access procedure on the D-channel (LAPD), when a selected one of the terminal equipment units is connected to the socket and is powered on to access the integrated services digital network and to set up a data link between the selected terminal equipment unit and the integrated services digital network, a request is at first issued from the selected terminal equipment unit to the integrated services digital network. Responsive to the request, the integrated services digital network searches for an unused or idle one of terminal endpoint identifiers (TEI) managed by the integrated services digital network and assigns the unused terminal endpoint identifier to the selected terminal equipment unit with the assigned terminal endpoint identifier retained or registered in the integrated services digital network.
Thus, an unused terminal endpoint identifier is preferentially assigned to each terminal equipment unit each time when the integrated services digital network is accessed. As a result, assigned terminal endpoint identifiers are successively registered on a registration table of the integrated services digital network.
Herein, it is to be noted that such assigned terminal endpoint identifiers are not eliminated from the registration table even when the terminal equipment unit is disconnected from the integrated services digital network after completion of communication. Under the circumstances, let a following one of the terminal equipment units issue a following or new request of assigning or granting a terminal endpoint identifier on condition that no unused terminal endpoint identifier remains in the integrated services digital network. In this event, the integrated services digital network manages and checks whether or not all of the terminal equipment units have been actually connected to the integrated services digital network. Specifically, if a certain one of the terminal equipment units is disconnected from the integrated services digital network, the integrated services digital network assigns, to the following terminal equipment unit, a terminal endpoint identifier which was previously assigned to the certain terminal equipment unit. On the other hand, when the certain terminal equipment unit has been connected to the integrated services digital network, a similar checking operation is carried out with regards to a different one of the terminal endpoint identifiers and is successively continued until an idle terminal endpoint identifier is found by the integrated services digital network. At any rate, the checking operation must be prosecuted in accordance with a terminal endpoint identifier assignment procedure prescribed in the CCITT I.441.
From this fact, it is readily understood that the integrated services digital network has a variable response time to a request of assigning or granting a terminal equipment identifier. Accordingly, a long time is wasted to set up a data link.